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Why we become nearsighted

This is the million dollar question.  First you need to know about the process called emmetropization.
What is emmetropization?
Emmetropization is the process by which the visual system adapts to imposed demand to eliminate blur.  This process is the heart of many visual problems including simple myopia, but it's also the solution.  The normal human eye has a specific geometry that focuses the image on the back of the retina.  Even slight changes to the optics, such as the lengthening of the eye by as little as 1mm produces a great deal of myopic blur (about 4D).  The optical dimensions of the eye must be almost PERFECTLY tuned to develop a clear picture.  Developmentally this is hardly possible without a feedback mechanism.  It's like ensuring that the right side of your body is PERFECTLY symmetrical to the left side of the body.  In theory, it is governed by the same genetic code, so it should be identical, but it never is due to local variations of growth signals.  Therefore, classical developmental biology can guide the formation of the eye structures, but it cannot set the perfect optical geometry to produce a clear picture.  In other words, the eye must "respond" to the quality of the picture on the retina and "adapt" to clear it.  This is increasingly important in young children due to the rapid growth rate of the eye.  To maintain a clear image throughout childhood, this mechanism must operate at a faster rate in younger people.  This mechanism is called "emmetropization" (emmetropia is a term referring to normal eyesight).  The bottom line is that in my opinion, clear eyesight is not genetically coded, but developed.  This is supported by many studies where rabbits/chickens/mice were raised in a closed environment developed myopia, and corrected it when released into an open environment.

By which method does emmetropization occur?
Emmetropization alters the refractive state of the eye through one of the following mechanisms:
1.  Corneal curvature
2.  Cilliary body (eye lens + muscle)
3.  Axial length of the eye (probably the most important one)
4.  The refractive index of the fluid inside the eye

How does the emmetropization mechanism sense blur and know how to correct it?
The following is a bit confusing.  Read it in its entirety before you judge it, and read it several times to understand what I am saying.  I have tried to use pictures, graphs, tables, interpretive dance... none very easy.  Just have to wrap your head around the concept.

When a normal eye looks at an object, it can focus near, midrange, and far (simplified model).  I believe that the eye has the tendency to favour midrange focusing, and it signals a change towards midrange focusing through intra-ocular pressure (IOP).  A long time ago, a scientist published a study in which he implanted microchips into the eyes of rhesis monkeys to sense IOP.  He found that the IOP increased when the monkeys focused near, changed to neutral when they focused midrange, and decreased when they focus far.  For the sake of argument, lets assume that the eye always favours neutral IOP.  When we spend a lot of time doing near work, the eye is chronically focused near, and the average IOP is high.  This leads to the gradual lengthening of the eye in the long run to bring the previously "near" (high IOP) focus to "midrange" (neutral IOP).  Now, since the eye is myopic, the whole focusing system has shifted towards the near environment.  What was previously midrange is now the "far" focus, what was previously near is now midrange focus, and what was previously too close to see is the near focus.  The eye is now "happy" because the person still spends most of the time doing near work, where the eye is now focused midrange, and at that focal state the IOP is neutral.  What do you think happened to the old far focus as a result of this change?  Since the new "far point" is in midrange, the previous (real) far point is outside the accommodative range of the eye and becomes a blur.  Here's a table: 
(keep in mind that in a normal eye, 'Real Distance' and 'Visual Accommodation' coincide)

Table 1 - Person with 2D myopia
 Real Distance  Visual Accommodation*   IOP
 ---unused*  near  high
 near (0.5m)  midrange  neutral
 midrange  far  low
 far  ---unable to focus  low
*Unused means that the focal point is behind the eye, so focusing 'near' will not produce any image
*Visual Accommodation represents distances at which the eye can focus
Blue represents dominant focusing distance (i.e. a computer user).

The change of refractive state towards myopia occurred through the same process that's designed to maintain the eye in emmetropic state: emmetropization.  This name of this process is misleading because it allows the eye to adapt to the imposed demand, which is not necessarily towards normal.  It responds to the environment, which in this case is "near". 

What corrective glasses do to us?
This is where a lot of controversy is.  Mainstream optometry does not believe minus lenses have any effect on long-term vision.  Here is what I think:
The minus lens is intended to correct myopia by counteracting the emmetropization movement towards myopia.  As explained before, the eye shifted towards myopia to put the intra-ocular pressure (IOP) to neutral as a result of increased near-work.  Once the eye has put the near work at midrange (neutral IOP), the eye is satisfied and WILL REMAIN AT THIS REFRACTIVE STATE SO LONG AS THE ENVIRONMENT REMAINS UNCHANGED.  If your computer is 0.5m away from your eyes, your eyes will develop a 1/0.5=2D myopia to put this distance at midrange focus (in reality this figure is more like -1D for several reasons, but this is what it looks like mathematically).  Your eyes will remain at -2D, and will not develop heavier myopia because...why would they?  It wouldn't make any sense a all.  Someone would need a serious genetic flaw to get myopia more severe than -2D.  Then how do we explain the fact that most people wearing glasses have myopia more severe than -2D.
When we put on distance glasses to correct 2D myopic blur (in this case a -2D prescription), it "resets" the virtual focus to normal, thereby tricking the eye into thinking it has normal eyesight.  The exact same scenario recurs as above (assuming person still spends most of the time doing near work) begin to focus near, the focus for the majority of your work is no longer midrange, the average IOP increases, the eye becomes more myopic to neutralize IOP, near work is now in midrange, and the eye developed another 2D of myopia (total 4D) to adapt to the near work, your distance vision blurs, and you need a heavier prescription.  The optometrist tells you your eyesight got worse, and gives you a -4D lens to correct your new refractive error, and the entire cycle recurs, and you get -6D glasses next time, then -8D, etc, etc....  By the time the refractive error reaches approximately -10D, the risk for several degenerate visual diseases increases substantially: retinal detachment, glaucoma (due to excessively high IOP -- this condition is almost exclusively found in myopes), and leads to legal blindness with or without corrective lenses. 
In reality myopia typically slows by around -6D.  The number itself is irrelevant because the age factor comes into play.  Typically, early teens develop myopia, which gets worse until you hit your 20's.  At this point, several things happen to stop myopic progression:
1.  Your emmetropization process is slowed (most important one)
2.  Typically people get a job and a life, and don't spend most of their time playing computer games (elimination of near environment).
3.  They are tired of getting increasing prescriptions, and learn to live with a bit of myopic blur (insurance only covers less frequent optometrist visits). 
In other words, whatever myopia you are able to earn by the time you are in your 20's, that's what you get for the majority of your life. 
In reality, emmetropization process is never completely haulted, its just slower and gets balanced by other risk factors.  This fact allows us to improve our vision.(See below)
Think about it, before the invention of lenses, there were very few young or middle-aged people who had myopia of more than -2D.  This number is important because after -2.5D you become pretty much incapacitated and legally blind without glasses.  because your eyesight becomes worse than 20/200.  This means that the myopic person sees at 20 feet the normal person ought to see at 200 feet.  (NOTE: In modern legal system, you are only considered legally blind if you cannot see 20/200 line with or without glasses).  In some areas of the world, the prevalence of moderate to severe myopia (>2.5D) is as high as 40%.  Imagine all those people without glasses early in history; almost half of the population would be blind.  This obviously did not happen.  The bottom line is that the person cannot naturally develop moderate or severe myopia without the aid of minus lenses.  I've had 3 optometrists, and they all told me myopia is genetic.  In most cases myopia is not genetic, it's acquired.  I do agree there may be some genetic factors, such as the rate of emmetropization and the elasticity of eye tissues, but we aren't "programmed" to be nearsighted.  That would be counterproductive.